Hierarchical security mechanism for dynamically assigning security levels to object programs

• US 4,104,721 A
• Filed: 12/30/1976
• Issued: 08/01/1978
• Est. Priority Date: 12/30/1976
• Status: Expired due to Term

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Markstein, Peter Willy, Tritter, Alan Levi
• Primary Examiner(s): Chapnick, Melvin B.
• Application Number:
  US05/755,899
• Time in Patent Office: 579 Days
• Field of Search: 364/200 MS File, 364/900 MS File
• US Class Current: 711/164
• CPC Class Codes: G06F 12/1491 in a hierarchical protection system, e.g. privilege levels, memory rings

Abstract

A computer system organization which allows a program to specify a predetermined security level for other programs which it invokes, while at the same time being subject to security restraints placed on it either by a higher priority level invoking program or by the operating system. A plurality of security levels organized as a hierarchy which may be established by both problem programmers, and the operating system are then controlled by the operating system. A program cannot change its previously assigned level. Only a higher level invoking program can make such an alteration. A new program'"'"'s security level indicator must be validated and then a protection code or `mask` of a predetermined size related to the security level must be validated. The system utilizes a plurality of special purpose bits in every data word which bits contain the protection field. Level indicators for the particular program determine the use of the protection field. A series of linking registers or a `Link Stack` having appropriate logic circuitry connected thereto is utilized for keeping track of the security level of all programs in a hierarchical sequence currently running on the system. The stack allows proper branching back to an originating program and prevents violation of security rules. The hardware additionally provides a mechanism for automatically checking each and every memory access, whether read or write, to assure that a correct protection field is present in each of the memory data words which is to be accessed or stored into.

Claims

1. An electronic computing system having a hierarchical security mechanism incorporated therein, said system including a memory for storing both instructions and data, an instruction execution unit for extracting series of instructions from the memory and executing same, and a processing unit for performing the data operations required by said instructions, said hierarchical security mechanism being dynamically actuable by application programs and including

means actuable by an application programmer for assigning a hierarchical protection level for each program instruction sequence included in an overall running program, wherein successive hierarchical protection levels have successively reduced privelege to access storage locations in said memory,

means for assigning a unique protection field to each such instruction sequence,

means for storing the particular protection field assigned to each instruction sequence in every storage location in said memory to which it is intended that access by said instruction sequence is to be permitted, and

means actuable during the running of a program for ascertaining that the protection field of each memory access matches the protection field of the instruction sequence which initiated said access, which comprises means for comparing the protection field of a requesting instruction with the stored protection field of the accessed location in memory and predicating access on a successful comparison, said comparison being a function of the hierarchical protection level of the requesting instruction.

2. A hierarchical security mechanism as set forth in claim 1, including means for detecting the magnitude of the hierarchical protection level assigned to each instruction sequence and means for examining a portion of the protection field whose size is directly dependent on such hierarchical protection level magnitude and means for causing a security violation indication if the required degree of protection field matching does not occur.

3. A hierarchical security mechanism as set forth in claim 2, including means for requiring that any application instruction sequence subjected by it to security validation requirements, cannot under any circumstances invoke any sub-application instruction sequence not subject to precisely the same security validation requirements as the calling sequence, but can nevertheless impose upon such sub-application instruction sequence additional security validation requirements.

4. A hierarchical security mechanism as set forth in claim 2, including means for specifying that a given instruction sequence is to branch to another instruction sequence or sub-application including means for automatically assigning a lower hierarchical protection level to the sub-application than the originating instruction sequence, and means for assuring that the protection field for the sub-application matches that of the calling instruction sequence and further requires that any memory access must match an additional portion of the protection field as specified by the hierarchical protection level indicator for that sub-application.

5. A hierarchical security mechanism as set forth in claim 4, including means for allowing a plurality of sub-application instruction sequences to be called within a single instruction stream including means for always assigning a lower hierarchical protection level to each sub-application than the calling instruction sequence and means for providing that, at termination of the sub-application instruction sequence, the system returns to the specific location in the original instruction stream where the sub-application sequence began.

6. A hierarchical security mechanism as set forth in claim 5, including means operable when a sub-application sequence request is detected to automatically store the hierarchical protection level for the calling instruction sequence and also store the precise location in memory where the next instruction of the original calling instruction sequence may be obtained upon termination of the called sub-application instruction sequence.

7. A hierarchical security mechanism as set forth in claim 6, including means operable when a sub-application sequence request is detected to automatically store the protection field for the calling instruction sequence whereby said protection field of the original calling instruction sequence may be obtained upon termination of the called sub-application instruction sequence.

8. A hierarchical security mechanism as set forth in claim 7, wherein said means for storing the protection field for the calling instruction sequence comprises a plurality of linking registers forming a link stack wherein each register contains a storage location for 1) a hierarchical protection level indicator, 2) a branch-back address and 3) a protection field and means for assigning such a register to each predetermined security level within said hierarchical security mechanism, and further including means to return to the branch-back address contained in one register of said link stack pointed to by another register in said link stack when a given sub-application instruction sequence is completed.

9. A hierachical security mechanism as set forth in claim 8, including a separate linking register in said link stack for each of the permitted levels of the security hierarchy and means for determining the hierarchical protection level field for any particular instruction sequence to automatically indicate the member of the link stack into which specific linking information is to be stored when it is required to link to that sub-application instruction sequence.

10. A hierarchical security mechanism as set forth in claim 9, including means operable when said computing system is operating in a multiprogramming mode to store the entire contents of the link stack at a predetermined location in memory whenever the time slot for a particular program running on the system terminates, or the said program is for any other reason interrupted, and means for recovering said entire link stack information from memory and restoring same to the linking registers whenever a time slot for said particular program is again allocated.

11. A hierarchical security mechanism as set forth in claim 9, including means for reserving the highest protection level to the operating system of the computing system, and means for utilizing the linking register location for said highest protection level as the working storage register for the hierarchical protection level indicator and the protection field against which all memory fetches must be checked.

12. A hierarchical security mechanism as set forth in claim 11, including means for preventing a sub-application program for returning to a hierarchical protection level of equal or less privilege than the level of the sub-application program from whence said return is being effected.

13. A hierarchical security mechanism as set forth in claim 12, including means for detecting that at least two sub-application programs are being specified by a given calling program at the same hierarchical level and means for assigning protection fields to each of said sub-application programs of said same hierarchical protection level whereby the memory space of each sub-application program will be protected from the other.

14. In an electronic computing system wherein said system includes a main memory for storing both instruction and data, an instruction execution unit for accessing and executing instructions from said main memory, an arithmetic and logic unit, and input/output devices selectively connectable to said system, wherein said system utilizes a data word format having a width including at least n bits which may be selectively utilized for security control purposes, the improvement which may be selectively utilized for security control purposes, the improvement which comprises a hierarchical security mechanism for controlling all memory accesses in said system, said mechanism including means for accessing an assigned predetermined hierarchical protection level and protection field for an instruction sequence currently running on the system wherein successive hierarchical protection levels have successively reduced privelege to access storage locations in said memory,

means for accessing the protection level and protection field assigned to a called instruction sequence, means for checking the protection level and protection field of said called instruction sequence to determine if 1) the called protection level is less than the calling protection level and 2) a specified portion of the two protection fields match before the called instruction sequence can proceed and means for producing a security violation if either the protection level or protection field do not match.

15. A hierarchical security mechanism as set forth in claim 14, including means for selectively assigning as many hierarchical security levels to instruction sequences to be run on said system as there are security control bits in the said protection field.

16. A hierarchical security mechanism as set forth in claim 15, including a hierarchical security mechanism link stack wherein said stack includes a plurality of linking registers wherein each linking register is dedicated to a particular hierarchical protection level and means for storing in each such register in a specified field, thereof, the level from which a particular running instruction sequence branched, the previously designated protection field for the calling instruction sequence, and the address in memory at which the next instruction in said calling instruction sequence is located.

17. A hierarchical security mechanism as set forth in claim 16, wherein the linking register for the highest protection level within said system is dedicated for use as a storage mechanism for the protection level and protection field of the instruction sequence currently running in the system, and means for comparing the contents of this register against the protection field of any memory access made to the main memory.

18. A hierarchical security mechanism as set forth in claim 17, including means for inserting a specified binary protection field in said highest level register of said link stack whenever a new instruction sequence begins to run on the system, said protection field being that of said new instruction sequence.

19. A hierarchical security mechanism as set forth in claim 18, including means operable to access a predetermined number of words in said main memory at an address specified by the operating system and inserting the protection field currently stored in said highest level register of said link stack in the security control bit field of such specific word in said memory.

20. A hierarchical security mechanism as set forth in claim 19, including means for allowing a programmer to request a particular security level and protection field from the operating system and for causing the operating system to assign said requested protection level and protection field to a particular instruction sequence and means for storing said protection level in the level and protection fields of said highest level register in said link stack and means for storing said protection level and protection field information in memory at an address specified by the operating system to become a necessary part of the instruction sequence for a particular program.

21. A hierarchical security mechanism as set forth in claim 20, including means for initiating a sub-application instruction sequence request by a calling instruction sequence including means for accessing the protection level of the sub-application instruction sequence, means for comparing said protection level with the level of the calling instruction sequence,

means for producing a security violation if the protection level of the sub-application instruction sequence is equal to or greater than the protection level of the calling instruction sequence, means for continuing with the branch instruction if the protection level of the sub-application sequence is less than the level of the calling instruction sequence, means for accessing the register of said link stack to which said sub-application protection level points, means for storing the protection level of the calling instruction sequence in the level field of said register, and means for storing the branch-back address and the protection field of the calling instruction sequence in said specified register of said link stack.

22. A hierarchical security mechanism as set forth in claim 21, including means actuable upon the termination of a sub-application instruction sequence to access the particular link stack register to which the level indicator for the currently running sub-application instruction sequence points, means for comparing the current protection level indication with the protection level indication in said pointed to link stack register and providing an interrupt if said called protection level is equal to or less than the protection level of the currently running sub-application instruction sequence, and means operable upon an indication that the level stored is larger than the current protection level to branch-back to the calling instruction sequence at the branch-back address stored in said link stack for storing the security level and protection field also stored in said pointed to link stack register in the level and protection fields of said highest level register of said link stack.

23. A hierarchical storage mechanism as set forth in claim 22, including means operable whenever a system interrupt occurs to load the entire contents of said link stack into main memory at a predetermined address and further means for recalling said entire link stack and reloading same into said linking registers when said particular instruction sequence which was interrupted is returned to by the system for execution.